1. Field of the Invention
The invention relates to a method of producing epoxysilicones via rhodium-based catalysts which promote a hydrosilation addition reaction between an ethylenically unsaturated epoxide and an organohydrogensilane or organohydrogensiloxane, without also promoting the oxirane ring-opening polymerization of the ethylenically unsaturated epoxide starting material or the epoxysilicone hydrosilation reaction product. The invention also relates to a curable epoxysilicone composition made by the present method.
2. Technology Review
In the production of epoxysilicone compositions, transition metal catalysts have long been known to promote the hydrosilation reaction. See, for example, J. L. Speier, "Homogeneous Catalysis of Hydrosilation by Transition Metals" in Advances in Organometallic Chemistry, Vol. 17, pp. 407-447, F. G. A. Stone and R. West, eds., Academic Press (New York, San Francisco, London), 1979; Aylett, Organometallic Compounds, Vol. 1, p. 107, John Wiley & Sons (New York), 1979; and Crivello and Lee, "The Synthesis, Characterization, and Photoinitiated Cationic Polymerization of Silicon-Containing Epoxy Resins", J. Polymer Sci., Vol 28, pp. 479-503, John Wiley & Sons (New York), 1990. Generally, the hydrosilation catalysts used are complexes of platinum, palladium, rhodium, iridium, iron or cobalt. In particular, platinum-containing catalysts have been widely used for this purpose.
The most commonly used platinum-containing hydrosilation catalysts are those derived from chloroplatinic acid. These catalysts tend to be unstable and to form metal cluster compounds or colloids (Cotton and Wilkenson, in Advanced Inorganic Chemistry, 4th edit., John Wiley and Sons (New York), 1980). Chloroplatinic acid itself is both thermally and photochemically unstable in solution. In addition, its composition is variable depending on its state of hydration. For example, chloroplatinic acid typically contains the H.sub.3 O.sup.+, H.sub.5 O.sub.2.sup.+, and H.sub.7 O.sub.3.sup.+ ions. On standing in solution at room temperature, chloroplatinic acid will oftentimes deposit elemental platinum. On thermal decomposition, volatile Pt.sub.6 Cl.sub.12 is also formed as one of the intermediates (Schweizer and Kerr, "Thermal Decomposition of Hexachloroplatinic Acid" in Inorganic Chemistry, vol. 17, pp. 2326-2327, 1978).
It has been found that in addition to catalyzing the hydrosilation reaction, many transition-metal-complex catalysts in the presence of silicon hydrides also promote the oxirane ring-opening polymerization of the ethylenically unsaturated epoxide starting material and the epoxysilicone product of the hydrosilation reaction. Reference is made, for example, to copending, commonly assigned application Ser. No. 07/473,802 (Riding, et al.), filed Feb. 2, 1990, which discloses the use of platinum or platinum-based catalysts to promote the oxirane ring-opening polymerization of epoxides. This ring-opening polymerization reaction during production of an epoxysilicone is undesirable as the epoxide polymerization may cause the reaction mixture to gel completely, resulting in the loss of the entire batch and in loss of considerable time in cleanup of the insoluble gelled resin.
Additionally, a partial gelation due to the ring-opening polymerization reaction can occur during epoxysilicone synthesis such that reproducible batch-to-batch viscosity of the epoxysilicone product is difficult to obtain. Such reproducibility in viscosity is highly preferred in the epoxysilicone industry, as these materials are typically used as coatings, for example release coatings, and the process of successfully and uniformly applying these coatings to a substrate is highly dependent upon the viscosity of the coating material. Commonly assigned, copending applications to Eckberg, et al., Attorney Docket No. 60SI-1466 and 60SI-1492, both filed Dec. 5, 1991, disclose that viscosity control can be achieved by use of a tertiary amine stabilizer during the hydrosilation synthesis reaction. However, only a limited number of transition-metal hydrosilation catalysts are active in the presence of this stabilizer.
In the presence of precious metal hydrosilation catalysts, epoxysilicones have been found to slowly gel on storage at room temperature due to the epoxide ring-opening polymerization reaction, thus shortening the shelf-life of the epoxysilicone product. While this storage problem can be partially alleviated by deactivating the transition-metal-complex catalyst with an inhibitor of its catalytic activity, such as dodecyl mercaptan or 2-mercaptobenzothiazole in the case of platinum complexes, it would be preferable to not incorporate this extra component and additional process step into epoxysilicone composition and production process.
In most of the catalytic systems involving platinum complexes, the catalytic species is not well understood. Recently, colloids have been shown to be the active species involved in some of catalytic hydrosilation reactions (Lewis, Journal of the American Chemical Society, vol. 112, p. 5998, 1990) and in the ring-opening polymerization of epoxides ("Novel Platinum Containing Initiators for Ring-Opening Polymerizations", Journal of Polymer Science, Pt. A; Polymer Chemistry Edition, Vol. 25, 1853-1863, 1991.) Other reports suggest that the catalytic species in the hydrosilation reaction is a non-colloidal metal complex (See, for example, Harrod and Chalk, in Organic Synthesis Via Metal Carbonyls, p. 673, Wender and Pino, eds., John Wiley & Sons (New York), 1977).
In order to minimize the oxirane ring-opening polymerization reaction, epoxysilicone fluids have been previously successfully produced only by careful control of batch temperature and olefin epoxide feed rate during the synthesis, followed by the above-mentioned inactivation of the catalyst after the completion of the hydrosilation reaction.
As disclosed in commonly assigned U.S. patent application of Crivello and Fan, entitled "Preparation of Epoxysilicon Compounds using Rhodium Catalysts", attorney docket 60SI-1374, certain rhodium-based hydrosilation catalysts selectively promote the hydrosilation reaction without the promotion of an epoxide ring-opening polymerization reaction. A variety of epoxy-containing silicone monomers and oligomers can be synthesized using these catalysts. However, most of the catalysts traditionally used for synthesis of epoxysilicone compositions, particularly Pt-containing catalysts, promote the epoxide ring-opening polymerization reaction, and therefore do not permit the selective hydrosilation synthesis of epoxysilicones.
The use of certain quaternary onium hexachloroplatinates as catalyst for the hydrosilation reaction between phenylacetylene and triethylsilane has been previously described. Reference is made to Iovel, I., Goldberg, Y., Shymanska, M. and Lukevics, E., in Organometallics, vol. 6, pp. 1410-1413, 1987. However, this study did not indicate the suitability of the quaternary onium hexachloroplatinates as useful hydrosilation catalysts for addition to vinyl epoxides, nor did it suggest that such salts effectively suppress the catalyst-dependent ring-opening polymerization of epoxy groups in either the starting ethylenically unsaturated epoxide or the epoxysilicone product of the hydrosilation reaction.
In consideration of the above, it is apparent that there exists a need in the epoxysilicone industry for a method of eliminating the oxirane ring-opening when employing commonly used hydrosilation catalysts. There also exists a need for an efficient yet economical method of producing epoxysilicone monomers and oligomers in the absence of the epoxide ring-opening side reaction, thereby generating epoxysilicone compositions of reproducible batch-to-batch viscosity. There is additionally a need for epoxysilicone composition which is stable to the epoxide ring-opening reaction and, therefore, has an increased shelf-life without the additional step and cost of poisoning the catalyst after the completion of the hydrosilation addition reaction.